As mobile devices have been continually developed and the demand for such mobile devices has increased, the demand for secondary batteries has sharply increased as an energy source for the mobile devices. Among such secondary batteries is a lithium secondary battery having high energy density and high discharge voltage, into which much research has been carried out and which is now commercialized.
In terms of the shape of batteries, the demand for prismatic secondary batteries or pouch-type secondary batteries, which are thin enough to be applied to products, such as mobile phones, is very high. In terms of the material for batteries, meanwhile, the demand for lithium secondary batteries, such as lithium ion batteries and lithium ion polymer batteries, which exhibit high energy density, discharge voltage, and output stability, is very high.
Based on the shape thereof, secondary batteries may be classified into a cylindrical battery cell, a prismatic battery cell, and a pouch-type battery cell. Among these kinds of secondary batteries, much interest is currently focused on the pouch-type battery cell, which can be stacked with high integration, has high energy density per unit weight, can be manufactured at low cost, and can be easily modified.
The pouch-type battery cell may also be referred to as a lithium ion polymer battery because an electrode assembly impregnated with a lithium electrolyte in the state in which a positive electrode and a negative electrode are thermally welded to a separator is mainly used as an electrode assembly mounted in the pouch-type battery cell.
FIG. 1 is an exploded perspective view typically showing a general structure of a conventional representative pouch-type secondary battery.
Referring to FIG. 1, a pouch-type secondary battery 100 includes a stacked type electrode assembly 30 having pluralities of electrode tabs 31 and 32 protruding therefrom, two electrode leads 41 and 42 respectively connected to the electrode tabs 31 and 32, and a battery case 20, in which the stacked type electrode assembly 30 is received in a sealed state such that the electrode leads 41 and 42 are partially exposed outside of the battery case 20.
The battery case 20 includes a lower case 21 having a concave receiving part 23, in which the stacked type electrode assembly is located, and an upper case 22 for covering the lower case 21 such that the stacked type electrode assembly 30 is received in the battery case 20 in a sealed state. The upper case 22 and the lower case 21 are connected to each other by thermal welding in the state in which the stacked type electrode assembly 30 is mounted therein to form a sealed portion 24.
FIG. 2 is a perspective view showing the sealed state of a conventional pouch-type secondary battery 110 having an electrode assembly 101 mounted therein. The entirety of side sealed portions 102 and 103, as well as sealed portions 104 and 105, from which electrode tabs 106 and 107 protrude, are thermally welded.
In order to secure the hermetical sealability and insulation resistance of a pouch-type secondary battery, it is very important to seal the pouch-type secondary battery. In particular, for a middle or large-sized battery, which requires a large internal capacity, the size of a sealed portion formed at the battery is increased. To this end, it is necessary to perform an additional process. For example, in order to reduce the unnecessary space of the battery, the remaining area of the battery is bent after the battery is sealed. However, the mechanical processing of the sealed pouch is not easy.
Therefore, there is a strong necessity for technology that is capable of solving the above problems and, in addition, solving problems that may occur when a sealed portion of a pouch-type secondary battery is bent.